1. Field of the Invention
Embodiments of the invention relate generally to nonvolatile semiconductor memory devices. More particularly, selected embodiments of the invention relate to a high voltage output circuit for a nonvolatile semiconductor memory device and a method for measuring an internal high voltage generated by the high voltage output circuit.
2. Description of Related Art
Semiconductor memory devices can be roughly divided into two categories including volatile memories and nonvolatile memories. Volatile memories tend to have faster performance compared with nonvolatile memories; however, volatile memories lose stored data when disconnected from an external power source. On the other hand, nonvolatile memories tend to have efficient performance while providing the additional benefit of maintaining stored data even when disconnected from an external power source. As a result, nonvolatile memories have continued to be an increasingly popular form of data storage for a variety of devices, including devices where power is limited or may be lost unexpectedly.
Examples of nonvolatile semiconductor memory devices include masked read only memory (MROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM), to name but a few.
Unfortunately, data stored in some forms of nonvolatile memory, such as MROM, PROM, and EPROM, cannot be readily updated. For example, MROM, PROM, and EPROM cannot be erased using electrical signals. On the other hand, data stored in EEPROM can be readily erased, read, and programmed using electrical signals. Because of this capability, EEPROM is among the more popular and commonly used forms of nonvolatile memory in existence today.
In addition, many data storage devices such as digital cameras, cellular phones, etc., are required to be compact in the size. As a result, system designers are interested in the development of EEPROMs occupying a relatively small amount of space. As a result, the size of EEPROMs has continued to decrease and the integration density of the EEPROMs has continued to increase.
One type of EEPROM with relatively good performance, size, and integration density is flash EEPROM. Because of these good qualities, flash EEPROM is commonly used to provide mass data storage or semi-permanent code storage for devices such as personal computers and portable electronic devices.
Flash EEPROM (hereafter “flash memory”) can be roughly categorized according to the organization and type of memory cells therein. Different categories of flash memory include, for example, NAND type flash memory, NOR type flash memory, and AND type flash memory.
FIG. 1 is a block diagram of a conventional nonvolatile semiconductor memory device. The device of FIG. 1 is described in further detail in U.S. Patent Publication No. 2002/0024330.
In FIG. 1, a flash memory comprises an input/output buffer 22, a memory cell array 10, an X decoder 12, a Y decoder 18, a register 14, a Y gate 16, a read/write circuit 20, a command decoder 24, a read/write/erase control circuit 26, a reference voltage (REF) generator 28 for generating a reference voltage, and HV generators 30 and 32 for producing internal high voltage.
FIG. 2 is a circuit diagram providing a more detailed view of the memory cell array shown in FIG. 1 and FIG. 3 is a sectional view illustrating a memory cell of the circuit diagram of FIG. 2.
FIG. 2 illustrates a NOR type memory cell array structure in which a memory cell transistor is coupled to every intersection of word lines WL1-WLn and bit lines BL1-BLn. Each memory cell transistor comprises a metal oxide semiconductor (MOS) transistor having a floating gate FG and a control gate CG. A drain node “D” of a memory cell transistor MC1 is coupled to a corresponding bit line BL1, a source node “S” is connected to ground, and control gate CG is connected to word line WL1.
Memory cell transistor MC1 has a sectional face illustrated in FIG. 3. Referring to FIG. 3, electrons are injected into a floating gate 7 of memory cell transistor MC1 in a program operation, or electrons stored in floating gate 7 are transferred to a substrate 2 in an erase operation. The program operation can be performed, e.g., using a conventional technique such as hot-electron injection, and the erase operation can be performed, e.g., using a conventional technique such as Fowler-Nordheim (F-N) tunneling.
In a conventional erase operation, a voltage of about 6-8 volts is applied to substrate 2 and a voltage of about −10 volts is applied to control gate CG. Under these conditions, a voltage difference between control gate CG and substrate 2 generates an electrical field on floating gate FG such that the electrons stored in floating gate FG are removed onto substrate 2. The erase operation lowers a threshold voltage Vt of memory cell transistor MC1. Where threshold voltage Vt is sufficiently lowered such that current flows through a channel region of memory cell transistor MC1 during a read operation, memory cell transistor MC1 is considered to be successfully erased. In most flash memory systems, and for purposes of this written description, it will be assumed that a memory cell transistor (or more simply “a memory cell”) having such an erased state is considered to store a logical “1”.
In a conventional program operation, a voltage of 0 volts is applied to source “S” and drain “D” and a voltage of about 10 volts is applied to control gate CG. Under these conditions, an electrical field is generated on floating gate FG such that electrons are transferred from the channel region of memory cell transistor MC1 to floating gate FG via hot-electron injection, thereby increasing threshold voltage Vt of memory cell transistor MC1. Where threshold voltage Vt increases to about 5-9 volts such that current is prevented from flowing across the channel region memory cell transistor MC1 during a read operation, memory cell transistor MC1 is considered to be successfully programmed. In most flash memory systems, and for purposes of this written description, it will be assumed that a memory cell transistor having such a programmed state is considered to store a logical “0”.
In a conventional read operation, a read voltage of about 4.5V is applied to control gate CG of memory cell transistor MC1, and source “S” and substrate 2 are both connected to ground. Under these conditions, current will flow or not flow in a corresponding bit line based on a program state of memory cell transistor MC1. Where threshold voltage Vt of memory cell transistor MC1 is greater than a reference value, substantially no current will flow through the corresponding bit line, indicating a program state of logical “0”. Otherwise, current will flow through the corresponding bit line, indicating a program state of logical “1”. According to the presence or absence of current flow, the corresponding bit line will assume a high voltage level or a low voltage level, respectively.
Although the operation of a single memory cell transistor MC1 has been described above, the structure and operation of other memory cell transistors illustrated in FIG. 2 have substantially the same structure and function as memory cell transistor MC1.
A NOR-type flash memory device having memory cell transistors such as those illustrated in FIGS. 2 and 3 typically requires a negative voltage or a high voltage greater than a power supply voltage of the device in order to perform program, erase, and read operations. In the example of FIG. 1, such voltages are produced by HV generators 30 and 32. HV generators 30 and 32 undergo a setting operation in a manufacture step performed immediately after a fabrication of semiconductor memory device. The setting operation typically includes a testing operation used to determine whether HV generators 30 and 32 generate output voltages with desired amplitudes and an adjusting operation used to adjust the amplitudes of the output voltages based on the testing operation.
Conventionally, the respective output voltages of HV generators 30 and 32 are sampled at multiple time points to achieve measurements of their respective amplitudes. Such multiple sampling may be required due to fluctuations in the amplitude of the output voltages over time. In general, the number of total repetitive measurements can be in the hundreds to thousands. Unfortunately, however, such repetitive sampling tends to increase the time required to perform the setting operation, and in some cases it may not produce accurate measurements.
In addition, other operating voltages such as a read voltage may depend on the accuracy of the output voltages of HV generators 30 and 32. For example, the amplitude of the read voltage may change in accordance with vibrations of an internal high voltage having an amplitude controlled by output voltages of HV generators 30 and 32.
The difficulty of accurately measuring the output voltages of HV generators 30 and 32 may perhaps be better understood by referring to an example of a conventional high voltage generator 100 illustrated in FIGS. 4 and 5. A typical method of testing an output of high voltage generator 100 is also described below with reference to FIGS. 4 and 5.
Referring to FIG. 4, high voltage generator 100 generates an internal high voltage Int. HV. High voltage generator comprises a control logic unit 40, an oscillator 42, a high voltage pump 44, and a high voltage regulator 46. Internal high voltage Int. HV is output through an output node connected to a pad 50. The output node is also connected to external test equipment used to measure internal output voltage Int. HV.
Where the level of internal high voltage Int. HV output from high voltage generator 100 vibrates at regular or irregular cycles as shown in FIG. 5, measurement values at respective measured times t1, t2 and t3 are individually different. That is, the measurement values change according to a vibration amplitude RB of internal high voltage Int. HV, causing a drop in reliability of an overall measurement. Because of this measurement variation, hundreds of thousands of measurements may be required in order to generate a reliable measure of the amplitude of internal high voltage Int. HV.